Icedove extension to reduce header size to one or two lines

compactheader adds a button to compact Icedove's header pane to a single
line or expand it again. It also allows the user to disable the new
buttons that Icedove 3 added to the header pane.
This add-on integrates the functionality of "RSS Linkify Subject"
by Alex Dedul. (https://addons.mozilla.org/en-US/thunderbird/addon/1704)